Oxygen-Consuming Zero-Gap Electrolysis Cells With Porous/Solid Plates

ABSTRACT

An electrolysis stack ( 53 ) with oxygen-depolarized cathodes ( 31 ) employs solid-plate anodes ( 38 ) and porous-plate cathodes ( 42 ). The stack ( 53 ) of electrolysis cells ( 29 ) (e.g, hydrogen-chloride or chlor-allkali cells) each include an ion exchange membrane ( 32 ) sandwiched between an anode conductor ( 34 ) and a permeable cathode ( 35 ); an oxygen-consuming gas diffusion cathode ( 31 ) is adjacent the cathode conductor of each cell. Between the anode conductor of one cell and the gas diffusion cathode of an adjacent cell there is a composite bipolar plate ( 51 ) including a solid plate ( 38 ) having channels ( 39 ) for conducing salt solution and product of the process; the bipolar plates also include a porous plate ( 42 ) having channels ( 43 ) for conducting oxidant adjacent the gas diffusion cathode and channels ( 49 ) connected to a source of liquid (such as water or dilute sodium hydroxide).

TECHNICAL FIELD

This invention relates to industrial electrolysis cells, such as may beused in the production of chlorine gas from hydrogen chloride or fromsodium chloride solutions, employing an oxygen-consuming cathode, with azero-gap structure, which employs solid plates adjacent the anode wherechlorine is evolved and porous plates adjacent the cathode electrodehaving passageways for liquid solution and having passageways foroxygen-containing gas.

BACKGROUND ART

Traditional production of chlorine employs a process in which theelectrolysis of brine (that is, sodium chloride and water) to formchlorine gas and sodium hydroxide (also known as caustic soda) utilizinghydrogen-evolving cathodes. In the year 2003, U.S. production ofchlorine was about 13 million tons and production of caustic soda wasabout 15 million tons, the production of which consumed about 10 GW(about 317 trillion BTUs) of electrical energy; this corresponds toabout two percent of the total electric power generated in the U.S.

In order to achieve greater energy savings, the hydrogen-evolvingcathodes can be replaced by oxygen-consuming cathodes to produce anenergy savings of as much as 30%, in what are sometimes calledoxygen-depolarized cells.

Additionally, other industrial electrolysis processes that employhydrogen-evolving cathodes can also realize an energy efficiency benefitby employing oxygen-consuming cathodes. Examples include theelectrolysis of hydrogen chloride to recover chlorine or theelectrolysis of hydrogen bromide solution to produce bromine.

The original oxygen-consuming chlor-alkali processes involves an anodechamber, through which brine is circulated and from which chlorine gasevolves, a cathode chamber through which oxygen is circulated, and asolution chamber between the other two chambers in which water isconverted to sodium hydroxide. This is typically referred to as thefinite-gap design. It is thought that the layer of sodium hydroxidebetween the oxygen electrode and the membrane unfavorably increases thecell resistance, and hydrostatic pressure of the sodium hydroxidesolution leads to non-uniform gas/liquid interfaces within the oxygenelectrode, which results in electrode flooding in some spots, and inleakage of the sodium hydroxide into the oxygen compartment in otherspots.

To overcome these difficulties, a zero-gap oxygen-consuming chlor-alkalielectrolysis cell was developed, as disclosed in U.S. Pat. No. 6,117,286and described with respect to FIG. 1 herein. The cell 11 is portionedinto an anode chamber 13 and a cathode chamber 14 by means of anion-exchange membrane 12. The cell has a mesh-form insoluble anode 15,which may comprise a conventional insoluble titanium electrode known asa dimensionally-stable anode (DSA), in intimate contact with theion-exchange membrane 12, on the side thereof adjacent the anode chamber13. A sheet-form hydrophilic material 16 is in intimate contact with theion-exchange membrane 12 on the side thereof adjacent the cathodechamber 14. The cell 11 also has a liquid-permeable oxygen gas diffusioncathode 17 in intimate contact with the hydrophilic material 16 on theside thereof adjacent the cathode chamber 14. A mesh-form cathodecollector 18 is connected to the oxygen gas diffusion cathode 17 so thatelectricity is supplied through the anode 15 and the collector 18, asshown by the plus and minus signs.

An inlet 20 receives saturated aqueous sodium chloride solution as wellas discharging the chlorine gas which is produced, and an outlet 19discharges the aqueous solution of unreacted sodium chloride. An inlet21 receives humidified oxygen-containing gas and an outlet 22 allowsdischarge of excess oxygen-containing gas as well as sodium hydroxideformed in the process. Sodium hydroxide is generated on the surface ofthe ion-exchange membrane 12 which faces the cathode chamber 14 anddescends in a dispersed fashion, especially due to gravity, within thehydrophilic material 16, which provides less flow resistance to thesodium hydroxide solution than would the cathode 17 itself. The sodiumhydroxide solution drips from the lower edge of the hydrophilic material16 and passes through the outlet 22. This avoids having the sodiumhydroxide solution residing in the oxygen gas diffusion cathode andimpeding the oxygen-containing feed gas from smoothly permeating throughthe cathode.

In patent application publication US 2005/0026005, instead of employinga hydrophilic sheet material 16 adjacent the oxygen gas diffusioncathode 17, the oxygen gas diffusion cathode is provided with acomposite layer of carbon-supported platinum and polytetrafluorethyleneon the side of the oxygen diffusion cathode facing the cathode chamber14. The purpose is stated to avoid generation of peroxide, theprecipitation of which as sodium peroxide would cause liquid flowmaintenance problems and damage to the membrane 12 and/or the oxygengas-diffusion cathode.

The aforementioned zero gap, oxygen consuming electrode chlor-alkalicells present challenging problematic conditions. For instance, therelatively high viscosity and strong corrosiveness of concentratedsodium hydroxide can impede the effective transport of reactants andproducts within the cathode and can damage the cathode. Although thestructure of the aforementioned patent publication tends to avoid localdry out of the cathode which promotes the formation of harmful peroxide,it is extremely difficult to maintain the balance of having the cathodefully saturated but not flooded at practical reactant stoichiometries,especially under a wide variety of commercial operating conditionsacross extended periods of time.

Additionally, other industrial electrolysis processes, such as theconversion of hydrogen chloride to chlorine with an oxygen-depolarizedcathode, face analogous problematic issues.

DISCLOSURE OF INVENTION

Objects of the invention include: maintaining proper liquid balance inany oxygen-consuming, electrolysis membrane cell; providing a gasdiffusion electrode which will maintain three-phase boundaries of gas,liquid and solid, that include oxygen, water/other solutions (such aswater/caustic-soda solutions), and the cathode catalyst/supportparticles; providing a gas diffusion electrode which effectivelytransports oxygen to the cathode catalyst layer while removing theliquid products away from the catalyst layer; provision of a gasdiffusion electrode in an electrolysis cell which assures that theion-exchange membrane remains well hydrated without local dryoutregions, while at the same time preventing flooding of the cathodecatalyst layer; a chlor-alkali cell which does not require a supply ofair that is humidified; electrolysis cells that are not adverselyaffected by a gradient of liquid pressure from the top of the cell tothe bottom of the cell; and improved electrolysis cells.

This invention is predicated in part on the discovery that the presenceof a dilute solution of sodium hydroxide, in a relatively uniformconcentration and pressure, across the entire gas diffusion cathodeenhances operation of the oxygen-consuming chlor-alkali cell. Theinvention is also predicated on the discovery that the oxygen reactantgas can be provided directly to the oxygen-consuming cathode withoutinterference from sodium hydroxide or other liquid flooding, byproviding channels for a recirculating flow of sodium hydroxide, water,or other water-containing liquid separate from channels for a flow ofoxygen-containing gas, such as air.

In accordance with the invention, oxygen-containing gas is flowed acrossthe gas diffusion layer of an oxygen depolarized cathode of anelectrolysis cell, and a liquid solution is flowed through channelsseparated from the diffusion layer by a porous plate to hydrate themembrane and remove excess water.

According to one form of the present invention, an electrochemicalapparatus includes a composite, bipolar plate disposed between the gasdiffusion cathode of one oxygen consuming electrolysis cell and theanode of an adjacent cell; the portion of the bipolar plate adjacent tothe anode is solid and contains passageways which circulate saltsolution (such as brine or halide acid solution) and recover the gaseousproduct (e.g., chlorine or bromine) that is produced. The other portionof the bipolar plate is highly porous and hydrophilic, having reactantair or oxygen channels in one surface which are disposed in intimatecontact with the oxygen-consuming gas diffusion cathode, and havingchannels in the opposite surface of said porous plate, through whichliquid (e.g., sodium hydroxide or water, in some embodiments) iscirculated, the liquid solution entering those channels at a desiredreduced concentration (which may typically be on the order of 32%).According to the invention, the pressure of the sodium hydroxide orother water-containing liquid circulating through the porous plate islower than the pressure of the air or other oxygen-containing gas; thispressure differential provides a driving force for liquid removal fromthe gas-diffusion electrode, which prevents the gas diffusion electrodefrom being flooded, while at the same time the electrode and theadjacent membrane is kept well hydrated by the liquid solutioncirculating through the plate.

The invention may be practiced as a mono-polar cell, if desired.

An oxygen-depolarized electrolysis cell according to the presentinvention provides the proper amount of hydration throughout the face ofthe entire gas diffusion cathode, removing excess liquid product wherenecessary and providing additional moisture where necessary, at variouslocations across the planform of the gas diffusion cathode. Similarly,since the oxygen, in accordance with the present invention, is presentedto the gas diffusion cathode through separate air channels, at apressure higher than that of the liquid solution, the presence of air atall areas of the gas diffusion electrode is ensured.

The invention may be practiced by providing solid and porous carbonplates in accordance with techniques which are customary in theproduction of solid plates and porous, hydrophilic plates for use infuel cells, particularly for use in proton exchange membrane fuel cells.

Other objects, features and advantages of the present invention willbecome more apparent in the light of the following detailed descriptionof exemplary embodiments thereof, as illustrated in the accompanyingdrawing.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional, side elevation view of a zero-gap, oxygenconsuming chlor-alkali cell known to the prior art.

FIG. 2 is a sectioned side elevation view of a hydrogen-chlorideelectrolysis cell in accordance with the present invention, withstippling indicating the porous portion of the bipolar plate and sectionlines indicating the solid portion of the bipolar plate.

FIG. 3 is a sectioned side elevation view of a chlor-alkali cell inaccordance with the present invention, with stippling indicating theporous portion of the bipolar plate and section lines indicating thesolid portion of the bipolar plate.

FIG. 4 is a multiple representation of the chlor-alkali cell of theinvention shown in FIG. 3, illustrating the simplicity of the repetitivestructure of a stack of electrolysis cells in accordance with theinvention.

MODE(S) FOR CARRYING OUT THE INVENTION

Referring to FIG. 2, a hydrogen-chloride electrolysis cell 29 employingan oxygen-consuming, gas-diffusion cathode 31 includes a conventionalion-exchange membrane 32 flanked by conductive anode and cathode screens34, 35. The screens 34, 35 may contain catalysts that promote therespective electrochemical reactions, numbered 1-3 hereinafter, atrespective electrodes. The anode may be a conventional DSA. On the anodeside, a solid (non-porous) plate 38 includes passageways 39 for thecirculation of hydrogen chloride solution and for the extraction ofchlorine gas which is produced by the electrolysis process. On thecathode side, a micro-porous, hydrophilic plate 42 has passageways 43for conducting non-hydrated, oxygen-containing gas, such as air, andpassageways 44 for circulating a water-containing liquid such as sodiumhydroxide or simply water.

The electrode conductors 34, 35 are respectively positive and negativeterminals (as indicated by the plus and minus signs 46, 47) and theseare connected to an appropriate source of direct current electrical (DC)power 48. With a hydrogen chloride solution, water, and air beingprovided to the cell 29, and with the anode and cathode conductors 34and 35 connected across the DC power 48, the reactions at the anode areshown by equations 1 and 2, and the reaction at the cathode is shown byequation 3.

At anode

4HCl→4Cl⁻+4H⁺  1.

4Cl⁻→+2Cl₂+4e-  2.

At cathode

O₂+4H⁺+4e ⁻→2H₂O   3.

A significant difference of the cell 29 compared to the prior art isthat the oxygen-containing stream (e.g., air) need not be humidifiedprior to being fed to the cell 29. This is because the hydrophilic poresin the porous plate 42 are filled with water (or a dilute hydrogenchloride solution) and provide a means to fully hydrate (saturate withwater) the gas in the cathode passageways 43. Keeping the cellwell-hydrated is critical since the membrane is a poor ionic conductorif it dries out and the lifetime of the membrane is significantlyreduced under dry conditions. Additionally, since the water in thepassageways 44 is circulated at a pressure below that of the gaspressure in the cathode passageways 43, any excess liquid water in thecathode 31 is removed by this pressure gradient. The removal of excessliquid water from the cathode is necessary since liquid water is bothproduced at the cathode, via reaction 3, and is transported to thecathode from the anode with the flow of protons via electro-osmoticdrag. If the cathode is flooded with water, even in local spots, thiswill prevent the access of oxygen to the cathode catalyst layer 35, andinstead of reaction 3 the following reaction can occur in these floodedregions:

4H⁺+4e ⁻→2H₂   4.

Obviously, this reaction is not desired from a safety perspective.Additionally, it decreases the efficiency of the oxygen-depolarizedcathode.

FIG. 3 is described using an HCl solution as the source of chlorineproduct; however, other halide acids, such as HBr, may be used.

Referring to FIG. 3, a chlor-alkali cell 29 employing anoxygen-consuming gas diffusion cathode 31 includes a conventional ionexchange membrane 30 flanked by conductive anode and cathode screens 34,35. The screens may have catalysts to promote reactions 5-8,hereinafter, on respective electrodes. On the anode side, a solid(non-porous) plate 38 includes passageways 39 for the circulation ofbrine (sodium chloride solution) and for the extraction of chlorine gaswhich is produced by the electrolysis process. On the cathode side, aporous, hydrophilic plate 42, passageways 43 for conductingoxygen-containing gas, such as air, and passageways 44 for circulatingsodium hydroxide solution, which enters the passageways 44 in a desiredconcentration, such as on the order of 32%, and leaves the passagewaysin a more concentrated solution, due to the formation of sodiumhydroxide by the electrolysis process.

The electrode conductors 34, 35 are respectively positive and negativeterminals (as indicated by the plus and minus signs 46, 47) and theseare connected to an appropriate source of DC power 48. With brine,sodium hydroxide, and air being provided to the cell 29, and with theanode and cathode conductors 34 and 35 connected across the source ofpower 48, the reactions at the anode are shown by equations 5 and 6, andthe reactions on the cathode are shown by equations 7 and 8.

At anode

4NaCl→4Cl⁻+4Na⁺  5.

4Cl⁻→2Cl₂+4e ⁻  6.

At cathode

2H₂O+O₂+4e ⁻→4OH⁻  7.

4Na⁺+4OH⁻→4NaOH   8.

A significant difference of the cell 29 compared to the prior art isthat sodium hydroxide in an appropriate solution strength is provided tothe cell, the solution thereby providing the desired water (reaction 7)to eventually provide sodium chloride (reaction 8). By providing asodium hydroxide solution, not only is water provided so that air neednot be moisturized as in the prior art, but the concentration of sodiumhydroxide solution, and therefore water, will be substantially uniformacross the planform of the gas diffusion cathode 31. This is achieved bythe porous plate 42 which is hydrophilic and allows passage of sodiumhydroxide solution through the porous plate 42 and air channels 43 toreach the surface of the gas diffusion cathode 31. Additionally, thepressure gradient provided across the porous plate 42 will remove anyexcess liquid water in the cathode, which will enable adequate oxygenaccess to support reaction 7 and prevent hydrogen evolution fromoccurring at the cathode via:

4Na⁺+4H₂O→4NaOH+4H⁺  9.

followed by reaction 4, hereinbefore.

Even in a low concentration of alkali solution, carbon dioxide willresult in the formation of carbonate salts that will precipitate out ofsolution; therefore, the oxidant-containing source (e.g., air) shouldfirst be scrubbed to remove CO₂, as is conventional. However, the airstream need not be pre-humidified, or saturated with water, as describedhereinbefore. The invention may be practiced in a mono-polar cell design(a single cell), or it may be practiced in a stack of cells. The cell 29of FIG. 3 is shown in FIG. 4 with an additional cell 29 to illustratethe ease of repeatability so as to form a stack 53 of chlor-alkali cells29. As easily seen in FIG. 4, the solid plates 38 and porous plate 42together comprise a composite bipolar plate 51. If desired, mono-polarcells could also be readily constructed by those skilled in the artusing the porous-plate cathode concept taught herein.

The configuration of the porous plate 42 may be very similar to similarporous, hydrophilic plates which are known in fuel cells, and typicallycomprise woven carbon sheets which are rendered hydrophilic by treatingwith tin, or by other known processes. The solid plate 38 may comprisesolid carbon, solid metal, or any other suitable material, such as aplastic with carbon or glass fibers, metal or the like. The porous platehas to be conducting in a bipolar plate configuration, and preferablyshould be conducting in a mono-polar configuration, since it is thethickest part and current must flow parallel to the plate direction. Theporous plate 42 can be constructed of a variety of materials that havebeen used as porous reactant gas flow field plates in fuel cellapplications or it may be a porous metallic plate made from powderedmetal.

The gas diffusion electrode 31 may be constructed of conventionalmaterials used in fuel-cell cathodes, such as porous carbon papers,cloths, or non-woven materials. Alternatively, the gas-diffusionelectrode could be constructed of a metal screen or a combination ofmetal and carbon. The cathode catalyst layer 35 may be constructed in amanner similar to state-of-the-art proton exchange membrane (PEM) fuelcell cathodes, which typically are a combination of catalyst andion-exchange ionomer. In addition to the ionomers in the catalyst layer,other polymers (e.g., PTFE) may be used as binders and/or to control thedesired hydrophobicity and porosity of this layer. The catalyst shouldbe one that promotes the oxygen-reduction reaction, which typicallyrequires a noble metal such as platinum or some platinum-based alloy.Preferably, this catalyst should not be adversely affected by thepresence of chlorine, which may be present in small amounts at thecathode. The catalyst layer is typically formed by mixing the catalystwith the polymers in solution and carefully casting the resultant inkonto the membrane, or some other suitable substrate, to obtain thedesired porous structure after the solvent(s) are removed byevaporation. The ion-exchange membrane 12 may be the same as those usedin conventional chlor-alkali membrane cells or in PEM fuel cells. Thesemembranes are typically fluorinated polymers with sulfonate groups toprovide the ionic sites, such as Du Pont NAFION®. These membranes areformed into thin films by extrusion or casting and they can bereinforced with other materials (e.g., fibers of expanded PTFE) toimprove their mechanical properties. The anodes can be constructed withconventional materials used in chlor-alkali and hydrogen-chlorideelectrolysis cells.

1. A method of operating an oxygen-depolarized electrolysis cell havingan anode and having a cathode including a water permeable gas diffusionelectrode, said method comprising: feeding a solution to the anode ofthe cell selected from (a) a salt solution and (b) a solution of halideacid; applying DC power between the anode and the cathode of the cell todrive electrochemical reactions in the cell to produce desired product;and recovering desired product from the cell; characterized by: flowingoxygen-containing gas through passageways on a side of a poroushydrophilic plate adjacent to the gas diffusion electrode of thecathode; and circulating a water-containing liquid solution throughpassageways in the porous hydrophilic plate which are separated from theflow of oxygen-containing gas.
 2. A method according to claim 1 whereinsaid step of flowing is further characterized by: flowing non-hydratedoxygen-containing gas.
 3. A method according to claim 1 wherein saidstep of flowing is further characterized by: flowing oxygen-containinggas which is not saturated with water.
 4. A method according to claim 1wherein said step of flowing is further characterized by: flowingoxygen-containing gas at a pressure which is lower than the pressure ofthe water-containing liquid.
 5. A method according to claim 1 furthercharacterized by: removing substantially all carbon dioxide from theoxygen-containing gas before flowing the oxygen-containing gas throughsaid cell.
 6. A method according to claim 1 further characterized by:said step of feeding comprising feeding a halide acid solution in water;and said step of circulating comprises circulating water.
 7. A methodaccording to claim 6 further characterized by: said step of feedingcomprises feeding hydrochloric acid.
 8. A method according to claim 6further characterized by: said step of feeding comprises feeding brine;and said step of circulating comprises circulating a dilute solution ofsodium hydroxide.
 9. An electrolysis cell (29) with anoxygen-depolarized cathode (31), comprising: a permeable anode conductor(34), a permeable cathode conductor (35), an ion exchange membrane (32)disposed between and contacting said conductors, a solid plate (38)having salt/product channels (39) adjacent to said anode conductor,configured to receive salt solution and configured to conduct product ofsaid salt/product channels, and an oxygen consuming, gas diffusioncathode (31) contacting said cathode conductor; characterized by theimprovement comprising: a porous, hydrophilic plate (42) having oxidantchannels (43), extending from a first surface thereof contacting saidgas diffusion cathode, configured to receive an oxygen-containing gas,said porous hydrophilic plate also having liquid channels (44),extending from a second surface thereof opposite to said first surface,configured to receive a water-containing liquid.
 10. An electrolysiscell (29) according to claim 9 wherein: a noble metal or noble metalalloy catalyst is disposed in said cathode conductor (35) adjacent tosaid membrane (32).
 11. A stack (53) of electrolysis cells (29)according to claim
 9. 12. An electrolysis cell (29) according to claim 9wherein: said salt/product channels (39) are configured to receivebrine; said liquid channels (43) are configured to receive a dilutesolution of water and sodium hydroxide; and said salt/product channelsare configured to provide chlorine as product.
 13. An electrolysis cell(29) according to claim 9 wherein: said salt/product channels (39) areconfigured to receive halide acid solution; said liquid channels (43)are configured to receive water; and said salt/product channels areconfigured to provide chlorine as product.
 14. An electrolysis cell (29)according to claim 13 wherein: said salt/product channels (39) areconfigured to receive hydrogen chloride solution.